Field of the Invention
Example embodiments relate generally to nuclear reactors, and more particularly to a method and apparatus for a fret resistant fuel rod for a Light Water Reactor (LWR) nuclear fuel bundle. The method and apparatus may include a fret resistant layer integrated within outer layers of fuel rod cladding, using embedded ceramic particles. The particles may be integrated within surfaces of the fuel rod cladding, by melting a thin layer of the cladding and re-solidifying the cladding to capture the particles within the cladding material matrix.
Related Art
As shown in FIG. 1, a conventional Boiling Water Reactor (BWR) nuclear reactor fuel assembly 10 includes a channel 12 with fuel rods 14 that may provide heat energy to a nuclear reactor to heat water into steam. While much of the discussion herein is directed toward a Boiling Water Reactor (BWR) fuel assembly 10, it should be understood that example embodiments may be applied to Light Water Reactors (LWRs) in general, including Pressurized Water Reactors (PWRs) and Canada Deuterium Uranium (CANDU) reactors. The steam is produced to cycle through steam turbines (not shown) to convert heat energy into work to ultimately produce electricity. Fuel rods 14 may be anchored in a lower tie plate 18, and may extend through spacers 22 to varying axial elevations within the assembly 10. For instance, full length fuel rods 14 may extend up to upper tie plate 20, and long partial length fuel rods 14a may extend just below the upper tie plate 20. Short part length fuel rods 14b may only extend just beyond the lowest level spacer 22. The fuel rods 14 contain nuclear fuel pellets 16 (as shown in more detail, in FIG. 2), and therefore integrity of the cladding 24 of the fuel rods 14 is critical to ensuring that the fuel 16 does not escape the confines of the fuel rods 14. Leaking fuel that escapes the confines of the fuel assembly 10, and migrates throughout equipment located within the BWR steam cycle, may cause costly BWR system maintenance and/or plant shutdown.
During operation, water and steam flowing through the fuel assembly 10 may frequently contain foreign material (debris) in the form of loose metal shavings, wires, and other materials which typically originate at reactor locations remote from the fuel rods 14. These materials may be sufficiently hard to wear or fret the soft fuel rod material (often made from a zirconium-alloy). During reactor operation, this debris can migrate into the opening in the lower tie plate 18 and enter the fuel bundle. Debris can also enter the fuel bundle through the upper tie plate 20 during refueling operations. Once inside the bundle, debris may be entrapped by the spacers 22 where it may be maintained in a quasi-suspended state (due to fluid flow). Debris may cause cladding 24 of each fuel rod 14 to be particularly susceptible to debris fretting, whereas the debris may cyclically contact the fuel rods, imposing wear forces sufficient to penetrate the fuel rod 14 walls. Severe wear forces may also be placed on portions of the fuel rods 14 that contact spacers 22 (this is particularly the case in PWRs, where Grid to Rod Fretting, or GRE, may be prevalent). Cladding 24 wear may further be caused during fuel assembly 10 manufacturing and maintenance, as the fuel rods 14 may contact other fuel assembly 10 components during insertion (and removal) of the fuel rods 14 into (and, out of) the channel 12 of the assembly 10.
Cladding 24 of fuel rods 14 is typically manufactured from a zirconium-alloy. The hostile environment of the reactor requires that structural modifications and/or material that is added to the fuel rod cladding 24 must satisfy a number of constraints. First, any wear resistant material added to the cladding must be approximately equal to or harder than the metallic debris particles found in the fuel assembly, to effectively resist abrasion from the particles. Second, any material applied to the cladding must be compatible with the thermal expansion of the cladding and form a strong bond with the cladding. Third, any material added to the cladding must be resistant to the chemical environment in the reactor, which characteristically includes hot water and steam in the case of BWRs and lithium hydride and boric acid in the case of PWRs. Fourth, the thickness of any material applied to the cladding must be relatively thin, so that the flow of water around the fuel rods is not significantly impeded. Fifth, any material added to the cladding is preferably capable of application in a process which does not require heating of the cladding tube above 400° C., to maintain the integrity of the cladding. Sixth, any material added to the fuel rod must not react with the cladding material or cause a reaction between the cladding and the environment.
Coatings of various forms and functions have conventionally been applied to fuel rod cladding, to provide a contiguous, dissimilar material layer to cladding to protect it from wear resistance. For example, a thin coating of an enriched boron-10 glass has been deposited on fuel rod cladding. Electroplating of fuel rod cladding has also been used, to provide a matrix metal and boron compound of, for example, nickel, iron manganese or chrome to coat the outside of the cladding. Furthermore, vapor deposition of volatized boron compounds have been applied to cladding. Lastly, ion-assisted vacuum deposition techniques, such as cathodic arc plasma deposition (CAPD), have been employed to deposit thin films on fuel rod cladding to increase wear resistance. Using each of these conventional methods, coatings or layers of wear resistant material form only a contiguous layer of protection that is not integrated within the actual cladding itself.